The present invention is generally directed to temperature control of ink jet print heads. The invention is more particularly directed to an arrangement of heater resistors on an ink jet print head chip for providing even distribution of heat across the chip.
An ink jet printer forms images consisting of patterns of ink dots. The ink dots are formed by droplets of ink that are ejected from an array of ink jet nozzles onto a print medium. The quality of the image formed by the ink jet printer is dependent, among other things, upon careful control of the volume and mass of the ink droplets. Ideally, the volume and mass of each droplet ejected from each nozzle in the array should be the same. Further. for best image quality, the volume and mass of ink droplets ejected from a single nozzle in the array should not vary over time.
One of the factors that affects ink volume is temperature. If there is significant variation in temperature from one area of the nozzle array to another, there is typically a corresponding variation in droplet volume. Thus, it is desirable to carefully control the temperature of an ink jet print head to keep the temperature fairly constant across the length of the nozzle array.
As the state of the art advances, ink jet printers are incorporating longer nozzle arrays to produce wider printed swaths. The latest print head designs are also incorporating metal heat sinks to transfer excessive heat away from the circuitry on the print head chip. Print heads having the longer nozzle arrays and/or metal heat sinks can develop significant temperature variations across the length of the nozzle array. As discussed above, such temperature variations across the array can have detrimental effect on the printed image.
Therefore, an apparatus is needed for reducing temperature variations across an ink jet print head.
The loregoing and other needs are met by an ink jet print head that includes a nozzle plate having a substantially linear array of ink jet nozzles through which ink droplets are ejected toward a print medium. An integrated circuit chip is disposed adjacent the nozzle plate on the print head. The chip includes a semiconductor substrate, a source voltage conductor disposed on the substrate and connected to a source voltage, and a ground return conductor disposed on the substrate. A substantially linear array of ink heating resistors are disposed on the substrate substantially parallel to the length of the chip. Each of the ink heating resistors is associated with a corresponding one of the ink jet nozzles. The chip also includes a plurality of substrate heater resistors disposed on the semiconductor substrate in a substantially linear arrangement and aligned substantially parallel with the ink jet nozzles. The substrate heater resistors are electrically connected in parallel with one node of each of the substrate heater resistors being connected to the source voltage conductor and another node of each of the substrate heater resistors being connected to the ground return conductor.
In preferred embodiments of the inventions the substrate heater resistors include first substrate heater resistors that are disposed near a lengthwise center of the chip and second substrate heater resistors that are distally disposed relative to the lengthwise center of the chip. The first and second substrate heater resistors have first and second electrical resistance values. respectively that are determined by thermal dissipation patterns of the chip. Preferably the second electrical resistance values are different from the first electrical resistance values. The difference between the first and second electrical resistance values cause the first and second substrate heater resistors to generate different amounts of heat when supplied with the source voltage. The different amounts of heat generated by the first and second substrate heater resistors and the relative positions of the first and second substrate heater resistors compensate for differing thermal dissipation patterns across the chip.
Thus, the present invention significantly reduces temperature variations across the print head chip and thereby reduces differences in ink temperature along the array of ink jet nozzles. By compensating for differences in ink temperature along the array of nozzles. the invention essentially eliminates temperature-induced variations in the sizes of ejected ink droplets.